In a mobile communication system, a User Equipment (UE) can receive information from a Base Station (BS) on a downlink and transmit information to the BS on an uplink. Data that the UE transmits or receives includes data and various types of control information and thus various physical channels are defined according to the types and usages of information transmitted to or received at the UE.
FIG. 1 illustrates physical channels and a method for transmitting signals on the physical channels in a mobile communication system, 3rd Generation Project Partnership Long Term Evolution (3GPP LTE).
Referring to FIG. 1, when a UE is powered on or enters a new cell, the UE performs initial cell search (step S101). The initial cell search involves acquisition of synchronization to a BS. Specifically, the UE synchronizes its timing to the BS and acquires a cell Identifier (ID) and other information by receiving a Primary Synchronization CHannel (P-SCH) and a Secondary Synchronization CHannel (S-SCH) from the BS. Then the UE may acquire information broadcast in the cell by receiving a Physical Broadcast CHannel (PBCH) from the BS. During the initial cell search, the UE may monitor a downlink channel state by receiving a DownLink Reference Signal (DL RS).
After the initial cell search, the UE may acquire detailed system information by receiving a Physical Downlink Control CHannel (PDCCH) and receiving a Physical Downlink Shared CHannel (PDSCH) based on the PDCCH (S102).
If the UE is yet to complete its connection to the BS, the UE may perform a random access procedure to complete the connection (S103 to S106). During the random access procedure, the UE may transmit a predetermined sequence as preamble on a Physical Random Access CHannel (PRACH) (S103) and receive a response message to the random access on a PDCCH and a PDSCH associated with the PDCCH (S104). In case of contention-based random access except for handover, the UE may perform a contention resolution procedure by transmitting an additional PRACH (S105) and receiving a PDCCH and a PDSCH associated with the PDCCH (S106).
After the above procedure, the UE may receive a PDCCH and/or a PDSCH from the BS (S107) and transmit a Physical Uplink Shared CHannel (PUSCH) and/or a Physical Uplink Control CHannel (PUCCH) to the BS (S108), which is a general downlink and uplink signal transmission procedure.
FIG. 2 illustrates an operation for processing an uplink signal for transmission at a UE.
Referring to FIG. 2, in the UE, a scrambler 201 may scramble a transmission signal with a UE-specific scrambling signal. A modulation mapper 202 modulates the scrambled signal received from the scrambler 201 to complex symbols in Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), or 16-ary Quadrature Amplitude Modulation (16QAM) according to the type and/or channel state of the transmission signal. A transform precoder 203 processes the complex symbols received from the modulation mapper 202. A resource element mapper 204 may map the complex symbols received from the transform precoder 203 to time-frequency resource elements for actual transmission. After being processed in a Single Carrier Frequency Division Multiple Access (SC-FDMA) signal generator 205, the mapped signal may be transmitted to a BS through an antenna.
FIG. 3 illustrates an operation for processing a downlink signal for transmission at a BS.
In the 3GPP LTE system, a BS may transmit one or more codewords on a downlink. As is done on an uplink in the signal processing operation illustrated in FIG. 2, scramblers 301 and modulation mappers 302 may process one or more codewords to complex symbols. A layer mapper 303 may map the complex symbols to a plurality of layers and a precoder 304 may multiply the layers by a precoding matrix selected according to a channel state and may allocate the multiplied signals to respective antennas. Resource element mappers 305 map the antenna-specific signals received from the precoder 304 to time-frequency resource elements. After being processed in Orthogonal Frequency Division Multiple Access (OFDMA) signal generators 306, the mapped signals may be transmitted through the antennas.
In the mobile communication system, Peak-to-Average Power Ratio (PAPR) may become a big issue to uplink signal transmission from a UE, relative to downlink signal transmission from a BS. Accordingly, SC-FDMA is adopted for uplink signal transmission, while OFDMA is used for downlink signal transmission, as described before with reference to FIGS. 2 and 3.
FIG. 4 illustrates SC-FDMA used for uplink signal transmission and OFDMA used for downlink signal transmission in the mobile communication system.
Referring to FIG. 4, both a UE and a BS commonly have a Serial-to-Parallel Converter (SPC) 401, a subcarrier mapper 403, an M-point Inverse Discrete Fourier Transform (IDFT) module 404, and a Cyclic Prefix (CP) adder 406, for uplink transmission and downlink transmission.
Notably, the UE further includes a Parallel-to-Serial Converter (PSC) 405 and an N-point Discrete Fourier Transform (DFT) module 402 to transmit an uplink signal. The N-point DFT module 402 is characterized in that it partially compensates for the effects of IDFT performed by the M-point IDFT module 404 such that a transmission uplink signal assumes a single carrier property.
FIG. 5 illustrates signal mapping methods in the frequency domain to satisfy the single carrier property in the frequency domain. Specifically, FIG. 5(a) illustrates localized mapping and FIG. 5(b) illustrates distributed mapping. Only localized mapping is allowed in the current 3GPP LTE system.
Now a description will be given of a modification of SC-FDMA known as clustered SC-FDMA. In clustered SC-FDMA, DFT output samples are divided into sub-groups and sequentially mapped to subcarrier areas which are spaced from one another for the respective sub-groups at the input of Inverse Fast Fourier Transform (IFFT) samples, during subcarrier mapping between a DFT process and an IFFT process. When needed, clustered SC-FDMA may involve filtering and cyclic extension.
The sub-groups may also be referred to as clusters and cyclic extension is to insert a guard interval longer than the maximum delay spread of a channel between successive symbols in order to prevent Inter-Symbol Interference (ISI) caused by multi-path propagation.